A modem commercial aircraft is very big. A Boeing 747-400, in its cargo configuration, for example, can hold aloft a cargo of 6,025 ft3 (170.5 m3), the equivalent of 30 LD-1 containers. Wings of just 211 feet 5 inches (64.4 m) hold a maximum dead-weight load of 875,000 lb (396,890 kg) aloft in flight. The construction of such a large and complex machine is a daunting task. Designers of such aircraft chose to divide the task into the construction of subassemblies, bringing the subassemblies into a whole later in the construction process.
There are several major subassemblies in the modem commercial airframe. The designers select the division lines between subassemblies to segregate assemblies into portions whose geometric configuration allows for rapid fabrication. They define these subassemblies to include complete groups of parts known as families.
A family of parts is any group of parts that do not vary greatly from one to the next, i.e. the dimensions of parts across the family remain relatively the same, one to the other, with variations in length, width or height. A good example of such a family would be wing ribs. The wing rib at the root of the wing and the tip of the wing are, basically, the same configuration, an airfoil. They vary only in length, width, and height, when compared to one another. Other such families include wing ribs, leading and trailing edge ribs on the wings and on the horizontal and vertical stabilizers, horizontal and vertical stabilizer ribs, body panels, fuselage frames, floor beams and miscellaneous subassemblies.
Manufacturers of modern airframes prefabricate numbers of subassemblies or workpieces on distinct fixtures known as jigs. These jigs exploit the similarity of the family of parts that comprise the assembly. To place and to fasten the various parts within the confines of the jig has traditionally required tooling to locate and to manually drill the holes necessary. Tooling for wing assembly, for example, is generally a plate of either metal or a composite of metal and various thermoplastics. Such plates are indexed either to a fixed point on the work piece or on the fixed assembly jig in order to precisely locate the sites for the holes. Additionally, the tooling or template guides the drill to the hole site in order to assure that the hole is drilled perpendicular to the surface. The maximum fastening strength occurs when the holes are precise circles. Slightly skewing the drill, away from perpendicular to the surface, results in oval holes that are larger than the inserted fastener. Extra space in the hole allows the fastened parts to work, that is, to move slightly. Working fatigues the joint.
Tooling, when new, does produce precisely located and properly angled holes. Placement of the tool on the jig or work piece causes wear on the indexing surfaces. Drilling in the tool causes wear within the hole on the template. Over time and with use, the wear becomes significant. The maximum wear that may be present without rendering the tool or template useless is termed tolerance.
The wear in a single tool or template does not complete the definition of the problem. In the traditional manner of assembly, each part has required its own tooling. In relating one part to another on the workpiece, the error introduced by inconsistencies of the tooling tends to mount up. The assembly technician who places part A on the workpiece does so with such errors as are inherent in the part A tool. Similarly, the assembly technician places part B with the errors inherent in the part B tool. The spatial relation of part A to part B is subject to both sets of errors. Because these errors cumulate, tolerances within a single tool or template must be much smaller than the total displacement of a hole site that might be acceptable.
In order to minimize the errors in part placement, the builder must subject the tooling to periodic calibration and maintenance. Continual calibration and maintenance of the figuring tooling, which can experience very hard usage in the factory, represents an immense expenditure of labor. Where such tooling indexes to the fixed assembly jig, as most tooling does, the calibration may include the calibration and routining of the fixed assembly jig. This calibration and routining of the jig prevents its use in production. Stopping production is expensive.
The problem is not trivial. An airplane requires a large number of holesxe2x80x94250,000 to 400,000 for fighter aircraftxe2x80x94two to three million for commercial airplanes. The Boeing 737-400 airplane requires holes to accommodate approximately 1,191,600,000 bolts and rivets; each fastener requiring drilling at least two surfaces, e.g. skin to ribs.
To respond to these shortcomings in traditional tooling, airplane manufacturers have sought to automate the drilling of these holes. Numeric definitions of the precise locations of hole sites come from the extensive drawings of the airplane stored in Computer Assisted Drafting (CAD) programs used to design the airplane. Airplane builders have sought several means of deriving precise locations for holes and then commanding a robotic driller to place them. Before the instant invention, such automated placement and drilling of holes has been limited in its success.
The most significant of the early approaches to the problem was the Huber, et al. U.S. Pat. No. 3,973,859, issued in Aug. 10, 1976. Huber teaches a monolithic automated drilling system. The Huber machine was, itself, as long as the work piece. Installation fixes this huge drilling machine in relation to the fixed assembly jig. The machine comprises a frame with freedom of movement along the length of the work piece (X-axis). Within the frame, the drilling spindle assembly can move upward and downward (Y-axis) and the spindle has an actuating piston that extends the drill toward the work piece (Z-axis). A spherical bearing carries the drill spindle with actuators and allows articulation in the either of two planes analogous to either pitch (A-axis) movement or yaw movement (B-axis) within the spherical bearing. This movement allows the positioning of the axis of the drill spindle perpendicular to the surface of the work piece.
While the Huber machine taught a very mobile drill spindle with means to position perpendicularly to the work piece surface, it was very large, very expensive, and incapable of accommodating changes to the working envelope. Out of necessity, fixing the Huber machine to floor of the construction facility maintains the Huber machines rough calibration to the fixed assembly jig. Because of this fixation, the builder accommodates changes in the work piece design by tearing down and rebuilding the machine in a location determined by the new design. Only one Huber machine can work on any one assembly of the airplane thereby limiting the speed of production.
The Murray, et al., U.S. Pat. No. 4,752,160, issued on Jun. 21, 1988 teaches an additional defect in the Huber machine. Murray refines the spherical bearing controlling xcex1- and xcex2-axes movement of the drilling spindle. In Huber, the spindle moves along the xcex1- and xcex2-axes by rotating around a point generally near the center of the spindle. Such movement displaces the point where the drill bit meets the work piece surface in the X-and Y-axes respectively. Murray refines Huber by shifting the point of rotation to that point where the drill meets the work piece surface. Murray accomplishes this shifting of the rotation point by employing large arcuate bearings that permit the cutting tool to rotate through a conical section having its vertex at the cutting point of the tool as a part of the spindle carriage. While cumbersome, the Murray machine mechanically achieves precision that the Huber machine cannot.
These means of automated drilling are still cumbersome, monolithic, inflexible, and expensive. Both Huber and Murray suggest that the solution will encompass control in as many as five-axes in order to assure roundness of drilled holes. Similarly, the mechanical means of controlling movement of the drill spindle such as the teaching of Murray, add to the complexity, weight and size of machines necessary for multi-axis numerically controlled or computer controlled drilling systems. However, prior art systems that incorporate computerized numerical-control have generally been highly expensive, fragile and difficult to hold in a proper position. Therefore, it should be apparent that a need has existed for a computerized numerically controlled drilling system that may be simply and accurately operated in multiple axes of motion and is smaller, more flexible, more easily deployable and more adaptable to changes in the design of the work piece. Such a driller could eliminate the need for completely separate drilling machines for each aspect of each assembly. It would also reduce the cost associated with fixture modification necessitated by assembly design changes, reduce the lead time to process new assemblies into production or introduce changes and reduce the level of skilled labor required to refer to drawings to set up parts in the fixtures and perform high-quality fastening.